The present invention relates to signal to noise ratio (“SNR”) estimators and methods for estimating the SNR of a received signal that includes a communication signal component and a noise component. The SNR estimate may be used to predict the bit error rate (“BER”) of the communication signal being received.
Current state of the art SNR estimators typically operate only within a very narrow range of signal and/or equipment parameters and therefore are limited to systems of a particular design. Such systems may require a particular modulation type, a constant signal envelope, a training sequence of bits, a defined signal structure, an auxiliary channel to pass required information for the SNR estimate, a stationary baseband signal, a linear receiver, etc. Additionally, most prior art systems are too power-hungry, due to the large number and high complexity of mathematical operations required to be performed, to operate in battery-powered equipment for any reasonable length of time. While prior art systems work well within these tightly-defined design requirements, these systems do not work well, and some not at all, if the design requirements are not adhered to.
While the above requirements appear to allow for a prior art SNR estimator to meet any given set of parameters, there are some combinations of parameters that cannot be met by the current state of the art. For example, there is a need for an in-service SNR estimator to operate on a signal with a non-constant envelope, non-linear modulation with memory, in a low power system with a non-linear receiver, and without training sequences and without auxiliary channels. No known prior art SNR estimator will meet these requirements.
Examples of prior art SNR estimators include those discussed in a paper by Pauluzzi and Beaulieu (“Pauluzzi”) entitled “A Comparison of SNR Estimation Techniques for the AWGN Channel” (IEEE Trans. on Comm., Vol. 438 No. 10, October 2000, pp. 1681–1691), the contents of which are hereby incorporated herein by reference. However, these systems are for baseband binary phase-shift keying (“PSK”) signals in real Additive White Gaussian Noise (“AWGN”) channels and use one or more of the following estimation techniques described in Pauluzzi to obtain an SNR estimate: split-symbol moment; maximum-likelihood; squared SNR variance; second-order and fourth-order moments; or signal to variation ratio. None of these systems will operate in a system with the afore-mentioned parameters.
A second paper, authored by Benedict and Soong (“Benedict”) entitled “The Joint Estimation of Signal and Noise from the Sum Envelope”, IEEE Trans. on Comm., Vol. IT-13 No. 3, July 1967, pp. 447–454, the contents of which are hereby incorporated herein by reference, describes a system for estimating SNR of an unmodulated sinusoid in an AWGN channel solely from envelope samples. The device disclosed in Benedict uses a maximum likelihood estimator in conjunction with a kurtosis method, which uses the second and fourth statistical moments. The Benedict device will not operate in a system with the afore-mentioned parameters.
A third paper, authored by Matzner and Englberger (“Matzner”) entitled “An SNR Estimation Algorithm Using Fourth-Order moments”, 1994 IEEE International Symposium on Information Theory, p. 119, the contents of which are hereby incorporated herein by reference, discloses an SNR estimating device which requires stationary signals and therefore will not operate in a system with the afore-mentioned parameters.
One embodiment of the present invention is an in-service, blind SNR estimator for a non-constant envelope, non-linear modulation signal with memory, in a low power system with a non-linear receiver, and operates without training sequences and without auxiliary channels. A blind estimator is one that has no knowledge of the signal being processed other than the signal's modulation while an in-service estimator derives an SNR estimate from the baseband, sampled, data-bearing received signal.
In another embodiment of the present invention, the problems of the prior art are avoided by estimating the SNR by a method which combines a signal power estimate with a signal-plus-noise power estimate. The signal-plus-noise power estimate may be obtained by time averaging the sum of the squares of the magnitudes of the power in the real (in-phase) and imaginary (quadrature) components of the received signal. The signal power estimate may be obtained by combining a demodulated bit sequence against samples of a delay discriminator output taken at an optimal sample time, and time averaging the result. The delay discriminator output may be produced by combining a time-delayed complex conjugate of the received signal with the received signal and then eliminating the real portion of the combined signal. The duration of the time delay may be less than the duration of a symbol in the received signal and may preferentially be approximately one-half of the symbol duration. The combining of the time-delayed complex conjugate signal and the received signal may be by any known means including, but not limited to, multiplication of the signals or correlation of the signals.
Although the above-described embodiment of the invention was designed to be used to meet the afore-mentioned parameters, the technique is not limited to those parameters but rather has applicability over a wide range of modulation formats, transmission channels, signal types, hardware constraints, etc. For example, nothing in the inventive system and method limits the applicability of the invention to any particular modulation format. The invention works with received signals modulated by formats including, but not limited to, frequency shift keying (“FSK”), constant-phase frequency shift keying (“CPFSK”), minimum shift keying (“MSK”), gaussian minimum shift keying (“GMSK”), phase shift keying (“PSK”), binary phase shift keying (“BPSK”), quaternary phase shift keying (“QPSK”), and quadrature amplitude modulation (“QAM”). The invention also operates with other modulation formats not mentioned above. Additionally, the invention is suitable for burst systems with non-constant envelopes.
The invention may be used in RF transmission systems such as satellite communication systems, cellular and PCS phones, software radios, local multipoint distribution systems (“LMDS”), multipoint multichannel distribution systems (“MMDS”), as well as fiber optic networks. Furthermore, the inventive SNR estimating technique can be used to determine communication link quality estimates for such applications as determining the bit error rate, power control, adaptive bit rate control, and fade detection. It is to be understood that the above-mentioned examples of modulation, transmission channels, and applications are exemplary only and are not intended to limit the invention or the applicability of the invention.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel system and method for an in-service decision-directed signal to noise ratio estimator.
It is another object of the present invention to provide a novel method for estimating the signal to noise ratio at a receiver of a transmitted signal having a modulated communication signal component including at least one data symbol and a noise signal component.
It is yet another object of the present invention to provide a novel method of estimating the SNR of a received signal having a modulated communication signal component and a noise signal component where the power of the received signal is compared with the power of the communication component of the received signal determined by a method including the step of combining the received signal with a delayed conjugate of the received signal.
It is still another object of the present invention to provide a novel SNR estimating method for a communication signal of either linear or non-linear modulation and either with or without memory.
It is a further object of the present invention to provide a novel SNR estimating method for either a constant envelope or non-constant envelope communication signal.
It is yet a further object of the present invention to provide a novel SNR estimating system and method for a communication signal modulated with any of the following modulation types: FSK, CPFSK, MSK, GMSK, BPSK, QPSK, or QAM.
It is still a further object of the present invention to provide a novel SNR estimating system and method where the power of the communication signal is derived from removing the real component of signal produced from the combination of the received signal and a delayed complex conjugate of the received signal.
It is an additional object of the present invention to provide a novel SNR estimating system and method where the power of the communication signal is derived from the imaginary component of signal produced from the combination of the received signal and a delayed complex conjugate of the received signal.
It is yet an additional object of the present invention to provide a novel SNR estimating system and method which operates in an in-service mode.
It is still an additional object of the present invention to provide a novel SNR estimating system and method which operates in a blind mode.
It is a further additional object of the present invention to provide a novel SNR estimating system and method which operates in a decision-directed mode.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.